A water-absorbent resin has been widely utilized in various fields such as hygienic materials such as disposable diaper and sanitary articles; horticultural materials such as water-retaining materials and soil improvers; and industrial and construction materials such as water blocking materials for cables and dewfall preventing materials. As the water-absorbent resins mentioned above, there have been known, for example, hydrolysates of starch-acrylonitrile graftcopolymers, neutralized products of starch-acrylate graftcopolymers, saponified products of vinyl acetate-acrylic ester copolymers, partially neutralized products of polyacrylic acid, and the like.
In recent years, an absorbent material in a hygienic material such as disposable diaper or sanitary napkin tends to be made thinner from the viewpoint of comfort upon use. When the absorbent material is thinned, the ratio of a water-absorbent resin in the absorbent material is increased, so that gel blocking of the water-absorbent resins with each other is likely to take place when a body fluid or the like is absorbed. In order to suppress the gel blocking of the water-absorbent resins with each other, it has been desired that the water-absorbent resins have a large amount of water absorption under pressure. In addition, a water-absorbent resin having a high water-absorption rate has been desired in order to prevent leakage of a body fluid or the like. Further, in order to maintain the comfort of the hygienic material upon a long-term use, a water-absorbent resin having a small amount of water-soluble substance has been desired.
In order to make the water absorption capability under pressure large, it is generally necessary to increase a crosslinking density of the water-absorbent resin, whereby the water-retention capacity of the water-absorbent resin is lowered. The absorption amount of the absorbent material using the above-mentioned water-absorbent resin is consequently lowered.
On the other hand, when a crosslinking density of the water-absorbent resin is lowered, an uncrosslinked component is increased, and upon contacting with a liquid, the water-absorbent resin is formed into lumps, whereby water absorption rate tends to be lowered, and a water-soluble substance is also likely to be eluted.
In order to solve the above problems, there have been known, a process comprising carrying out an aqueous polymerization in the presence of phosphorous acid and/or a salt thereof, to give a precursor of a water-absorbent resin, and thereafter mixing the precursor of a water-absorbent resin with a surface crosslinking agent while heating (see Patent Publication 1), and a process comprising carrying out a reversed phase suspension polymerization in the presence of a hypophosphorous acid, to give a precursor of a water-absorbent resin, and thereafter subjecting the water-absorbent resin to a surface crosslinking (see Patent Publication 2), and the like.
However, the water-absorbent resins obtained by these processes have been inferior in at least one property of water-retention capacity, water absorption capacity under pressure, water absorption rate, and the amount of water-soluble substance and have had a disadvantage that the water-absorbent resins cannot keep the properties satisfying all of the above. Therefore, a water-absorbent resin which is excellent in all of water-retention capacity, water absorption capacity under pressure, water absorption rate, and the amount of water-soluble substance is desired.
Patent Publication 1: Japanese Patent Laid-Open No. Hei 9-124710
Patent Publication 2: Japanese Patent Laid-Open No. Hei 2-255804